180 INVESTIGATION INTO ORIGIN OF SLEEHNG-SICKNEJSS 



" The history o£ a great proportion of the infected natives fails to show 

 that there had previously been any possibility of infection having taken 

 place in a known palpalis area. There is in fact reliable evidence in many 

 of the cases to justify any possibility of infection from a \uio\y\i ijalpalis area 

 being disregarded. 



" G. palpalis has not been found on any part of the river by Dr. Leach 

 after a very cnreful search, nor by Dr. Leach and many other observers in 

 the case of that portion of the river between Chewanda's and Hargreaves. 

 It is, however, notable that those cases of the disease which have been found 

 occurred in close proximity to the river or its tributaries^ in parts where 

 conditions were more or less favourable for harbouring this species, and 

 that no cases were found in parts where conditiojis were unfavourable, 

 although in both cases careful search failed to reveal the presence of 

 this fly. 



'^ Dr. Leach has advanced a theory which tends to fit in with these facts, 

 but which has not as yet been fully inveitigated. He suggests that in the 

 main stream at the time of the year at which his recent investigations were 

 carried ouf", while all the conditions as to shade, etc., suii?able for harbouring 

 palpalis were present, the water level was at that time so high as to very 

 much limit the extent of suitable breeding ground, and for that reason the 

 fly may have migrated to some of the tributaries which have as yet only 

 been examined for a short distance from their openings into the main stream. 

 It seems probable that for this, or some other reason, the fly may migrate at 

 certain seasons from the main stream to its smaller tributaries, and this 

 would account for the negative results of Dr. Leach's and Mr. Silverlock'g 

 search. 



" The possibility of infection being conveyed by some species other than 

 G. palpalis must not be lost sight of, and should a further search at a 

 different season of the year fail to reveal the presence of this species, it will 

 then be desirable to institute experiments, as suggested by Dr. Bagshawe 

 (Sleeping Sickness Bulletin, no. 17), to test whether any other biting- 

 flies which exist in these localities may be capable of transmitting the 

 disease." 



It is notable that in these reports no mention is made of Glossina fiisca, 

 although this large species has been recorded as occurring in the vicinity of 

 Hargreaves (Sleeping Sickness Bulletin_, no. 16, p. 146). But the fact that 

 this insect was not seen by Dr. Leach or Mr. Silverlock does not necessarily 

 constitute clear evidence as to its non-existence in the route which they 

 followed ; for while p)alpalis is a species of markedly obtrusive habits, it is 

 well known that fusca is very liable to be overlooked by collectors (unless 

 specially searched for), owing to its unusual habit of concealing itself during 

 the day. 



With regard to Dr. May's remark that the cases of sleeping sickness in 



